1. Field of the Invention
The present invention relates to a semiconductor device which has a circuit composed of a thin film transistor (hereinafter referred to as a TFT) and a manufacturing method thereof. The present invention relates to, for example, an electro-optical device typified by a liquid crystal display panel and an electronic device in which such an electro-optical device is mounted as a component.
Note that in this specification, a semiconductor device indicates, in general, a device which functions by utilizing characteristics of a semiconductor, and an electro-optical device, a semiconductor circuit, and an electronic device are each a semiconductor device.
2. Description of the Related Art
In recent years, the development of a semiconductor device has progressed. The semiconductor device has a large area integrated circuit manufactured by using a thin film transistor (TFT) composed of a semiconductor thin film (about several nm to several hundred nm in thickness) formed on a substrate having an insulating surface. An active matrix liquid crystal module is known as a typical example thereof. In particular, since a TFT using a crystalline silicon film (typically, a polysilicon film) as an active layer (hereinafter referred to as a polysilicon TFT) has high field effect mobility, it is also capable to form a circuit with various functions.
For example, in a liquid crystal module mounted in a liquid crystal display device, a pixel circuit for displaying an image in each functional block and a driver circuit for controlling the pixel circuit, which includes a shift register circuit, a level shifter circuit, a buffer circuit, a sampling circuit, and the like using CMOS circuits as fundamental circuits, which are formed on a single substrate.
Furthermore, in the pixel circuit of the active matrix liquid crystal module, TFTs (pixel TFTs) are arranged in each of the several tens to several millions of pixels, and a pixel electrode is provided in each of the pixel TFTs. A counter electrode is provided at a counter substrate side with sandwiching liquid crystals to form a kind of capacitor using the liquid crystals as a dielectric. Then, voltage applied to each of the pixels is controlled by a switching function of the TFT to control a charge to the capacitor and drive the liquid crystals, and the amount of transmitting light is control to display an image.
The pixel TFT is made from an n-channel TFT and is used as a switching element for applying voltage to liquid crystals and driving. Since the liquid crystals are driven by an alternating current, a method called a frame inversion drive is employed in many cases. In this method, in order to suppress the power consumption, it is important to sufficiently reduce an off current value (drain current flowing when the TFT is in an off operation state) as a characteristic required for the pixel TFT.
A low concentration drain (LDD: lightly doped drain) structure has been known as a structure of a TFT for reducing an off current value. According to this structure, a region to which an impurity element is added at a low concentration is provided between a channel formation region and a source region or a drain region which is formed by adding an impurity element at a high concentration, and the region is called an LDD region. An LDD structure is effective in preventing deterioration due to hot carrier injection by relaxing an electric field in the vicinity of a drain.
In the case where an LDD region is formed using a conventional TFT, an off current value could be reduced. However, an on current value also decreased.
Further, a GOLD (gate-drain overlapped LDD) structure in which an LDD region and a gate electrode are arranged so as to overlap with each other through a gate insulating film is known as means for preventing deterioration of an on current value due to hot carriers. Compared to an LDD structure, the GOLD structure is much more effective in preventing deterioration due to hot carrier injection by relaxing the electric field in the vicinity of a drain. Note that in this specification, a TFT structure in which a gate electrode and an LDD region overlap with each other through a gate insulating film is called a GOLD structure and a TFT structure in which a gate electrode and an LDD region do not overlap with each other through a gate insulating film, is called an LDD structure.
Although the GOLD structure is more effective in preventing deterioration of an on current value compared with an LDD structure, on the other hand, there is a problem that an off current value becomes large in the GOLD structure compared with the LDD structure.
Further, since an LDD region and a gate electrode are arranged so as to overlap with each other through a gate insulating film in the GOLD structure, a parasitic capacity occurred and the frequency characteristic (called f characteristic) decreased, thereby obstructing high speed operation.
In addition, in the case where a storage capacitor is formed in the pixel portion using a semiconductor layer as one of electrodes, a problem of leak current becomes greater.
As described above, in a semiconductor device which has a plurality of integrated circuits such as an active matrix liquid crystal display device, the above problem is more emphasized particularly in a crystalline silicon TFT as the performance required for the active matrix liquid crystal display device is enhanced.